During the operation of an electrochemical cell, such as a metal-air cell, gasses are released during the electrochemical reaction. These gasses must be vented from the interior of the case housing of the cell or cell operation and efficiency may be compromised. It is also desirable to prevent the passage of liquids into or out of the cell and to prevent the intrusion of contaminates while the internal gasses are being vented from the cell.
Metal-air cells include an air-permeable cathode and a metallic anode separated by an aqueous electrolyte. During the operation of a cell such as a zinc-air cell, oxygen from ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode and reacts with these hydroxide ions such that water and electrons are released to provide electrical energy. Various gasses are released within the cell structure during this electrochemical reaction causing the internal pressure in the cell case to increase with continued use. Because the cathode is usually not capable of supporting high hydrostatic pressures (typically less than 2 psi), the gasses generated within the cell case must be vented at low pressures to protect the cathode.
While venting internal gasses is possible through mechanical devices that can open and close to the atmosphere, these devices must reseal each time after venting. The hermeticity of the case may be sacrificed by the opening and closing a mechanical seal. The control of electrolyte leakage and equilibrium vapor pressure also may be difficult depending upon the size of the opening as well as the length of time in which the mechanical seal is open. Environmental contaminants, such as carbon dioxide, also may enter through the opening. Further, the relative humidity of the ambient air that enters the cell through the opening is also of concern. If the relative humidity of the ambient air is too high, then the battery may fail due to a condition called flooding. However, if the relative humidity of the ambient air is too low, then the battery may fail due to drying out.
Known methods for venting gasses generated from within a cell include that described in commonly owned U.S. Pat. No. 5,362,577, issued Nov. 8, 1994, entitled "Diffusion Vent for a Rechargeable Metal-Air Cell," disclosing a vent system providing at least one gas exit hole that is sufficiently small to prevent electrolyte leakage and also to prevent the intake of excess carbon dioxide or excess water vapor from the atmosphere. The disclosure of U.S. Pat. No. 5,362,577 is incorporated herein by reference. Generally, this invention also discloses the use of combinations of gas permeable, hydrophobic membranes, such as polypropylene, and diffuser materials, such as polyethylene, to cover the gas exit hole. A recess also may be provided within the case such that the gas exit hole communicates between the atmosphere and the recess. A gas collection area is defined by the recess formed in the case wall or by the gas diffuser membrane attached to the case.
In a preferred embodiment of the invention described in U.S. Pat. No. 5,362,577, the case has at least one recess defined on its interior surface. The recess extends towards the exterior of the case so as to define a gas collection area, with at least one gas exit hole communicating with the atmosphere. The gas exit hole has a smaller cross-sectional area than the cross-sectional area of the recess. The gas exit hole is covered with a gas permeable, hydrophobic membrane. A gas diffuser is retained or attached within the recess. A second gas permeable, hydrophobic membrane is then attached to the interior surface of the case and covers the gas diffuser and first gas permeable, hydrophobic membrane. The gas diffuser both supports the inner membrane when under pressure and laterally diffuses the gas between the membranes. The membranes are ultrasonically welded to the case.
While the system of U.S. Pat. No. 5,362,577 provides superior venting of exhaust gas from a battery case while maintaining a hermetic seal, what is needed is a simplified venting system. This simplified system would provide venting for the case while eliminating the need for some of the materials and construction techniques currently used. It is also desirable for a vent system to prevent the ingress of gasses, such as oxygen, that may corrode the anode and otherwise impede the operation of the cell. Further, because the anode tends to expand during discharge, it is desirable to provide structural support for the cell to ensure that the anode does not block the vent. These additional goals must be accomplished while maintaining an adequate venting system and insuring the hermeticity of the cell.